1. Field of the Invention
The present invention relates to a developer for electrostatic images which is used in the formation of an image by electrophotography, more specifically to a developer for electrostatic images which is adapted to a reprographic system wherein a magnetic brush development method is adopted, and particularly to a developer of two-component type which is used in a magnetic brush development system.
2. Prior Art
In electrophotography, a uniform surface charge is given in the dark to a photosensitive member comprising a photoconductive element, which is then exposed to light to form thereon an electrostatic image, which is thereafter converted into a visible image with a developer.
Methods of developing such an electrostatic image can be broadly classified into a liquid development system and a dry development system.
The liquid development system is a method wherein development is effected with a liquid developer comprising a fine powder of one of various pigments and dyes dispersed in an insulating organic liquid, while the dry development system is a method wherein use is made of a charged colored powder, usually called a "toner," comprising a dye or a pigment, such as carbon black, contained in a natural or synthetic resin.
In the latter system, a toner is charged to a polarity reverse to that of the charge of an electrostatic image and the charged toner is electrostatically adhered to the electrostatic image to form a visible image.
The dry development system include a method wherein use is made of a so-called single-component developer comprising only the above-mentioned toner as the main component, and a method wherein use is made of a so-called two-component developer comprising a carrier such as an iron powder or glass beads mixed with the above-mentioned toner.
The former corresponds to a charged toner contact development method (see U.S. Pat. No. 2,811,465), a powder cloud method [see Photo Eng., 6 (1955)3, and the like, while the latter corresponds to a magnetic brush method (see U.S. Pat. No. 2,786,439), a cascade method (see U.S. Pat. No. 2,618,551), and the like.
A visible image formed by adhering a toner to an electrostatic image according to the aforementioned development system is then fixed either as such on a photosensitive material or after transferred to an image support such as paper.
A fixing or fixation method such as oven fixing, flash fixing, heat roller fixing, pressure fixing and heat plate fixing have been proposed and employed. The heat roller fixing has been very often employed, among them, because miniaturiazation and a good heat efficiency are available. good in heat efficiency.
Among the above-mentioned development methods, a detailed description will now be made of two-component developers in connection with the magnetic brush method, to which the present invention pertains.
Carriers which have heretofore been used in the magnetic brush development method include an reduced ore iron powder prepared by reducing an iron ore, a reduced mill scale iron powder prepared by reducing mill scales, a spherical atomized-iron powder prepared by cooling and pulverizing molten steel flowed out of fine orifices, and an iron nitride powder prepared by nitriding thin steel pieces, pulverizing resulting nitride pieces and denitriding the resulting powder. Further, use has been made of a ferrite carrier which is obtained by granulating, drying and firing a ferrite powder comprising Fe.sub.2 O.sub.3 as the main starting material. Since an iron powder carrier is oxidized with moisture in the air to form Fe.sub.2 O.sub.3, namely rust on the surfaces thereof, it is forcibly oxidized to cover the surfaces thereof with thin stable oxide layers having a relatively high electric resistance. The electric resistance of this carrier can be controlled according to the degree of oxidation. When an iron powder carrier is adequately controlled in shape, particle size distribution and surface resistance, a good image having a high density can be obtained.
On the other hand, a ferrite carrier is characterized in that it is 30 to 40 % lower in specific gravity than iron powder carriers, can be widely varied in electric resistance and magnetic characteristics, is spherical to be good in flowability, and can be decreased in residual magnetization. Therefore, the ferrite carrier is adapted to prolongation of the life of a developer, which, however, is not on such a level as to meet the demand. Meanwhile, a resin-coated carrier prepared by forming resin coating layers on the surfaces of core particles of an iron powder carrier, a ferrite carrier or the like was developed and has recently attracted attention because it can prevent destabilization of the charge of a toner due to sticking of the toner on the surfaces of the carrier particles, has an excellent durability, and can control the frictional chargeability of the toner and reduce the dependence of the chargeability characteristics on environment.
The requirements of the resin-coated carrier include sufficient abrasion resistance and heat resistance of the coating layers on the surfaces of the core particles, a sufficient adhesion of the coating layers to the core particles, a good sticking inhibiting property of the coating layers to prevent toner particles from sticking to the surfaces of the carrier particles, and a capability of readily giving the toner desired level and polarity of chargeability. Specifically, in a developing apparatus, the resin-coated carrier particles undergo friction onto only with each other but also with toner particles and the wall of the apparatus. If the coating layers are abraded by such friction, the charging to be brought about by friction thereof with the toner is destabilized. Further, when the adhesion of the coating layers to the core particles is insufficient, the coating layers are separated from the core particles by the above-mentioned friction to lose the stable frictional chargeability. Furthermore, when the toner sticks to the resin-coated carrier, the frictional chargeability of the toner is spoiled.
Although various resin-coated carriers have heretofore been developed, none of them can fully satisfy the aforementioned requirements. For example, when a common resin is used as a material of coating layers, it is liable to cause sticking of a toner because of the high surface energy thereof. As a countermeasure thereagainst, it is conceivable to use a fluororesin having a low surface energy. Since, however, fluororesins are poor in adhesion and insoluble in almost all solvents and involve complicated coating and heat treatment methods, they are judged to be inadequate as coating materials for core particles.
Silicone resins can be mentioned as other resins having a low surface energy. Although they have advantages of water repellency and high resistance in addition to the low surface energy, they have a disadvantage of such poor adhesion that they are apt to cause separation when used in coating layers.
In order to overcome this disadvantage, there have been proposed various methods such as use of a resin-modified silicone resin (see Japanese Patent Laid-Open No. 127,569/1980), incorporation of a vinylsilane to be reacted with other resin (see Japanese Patent Laid-Open No. 32,149/1981), use of a mixture of a trialkoxysilane with ethylcellulose (see U.S. Pat. No. 3,840,464), and use of a mixture of an organosilicone terpolymer with a polyphenylene resin (see U.S. Pat. No. 3,849,127). However, these methods involve problems such as the necessity of a temperature as high as 300.degree. C. or above for formation of coating layers and/or the poor compatibility of a silicone resin with other resin which results in formation of non-uniform coating layers to fail to provide expected characteristics. Further, formation of coating layers at a comparatively low curing temperature has been proposed (see Japanese Patent Laid-Open No. 127,569/1980), but it provides insufficient adhesion and toughness of the coating layers, which is, therefore, liable to be readily abraded, leading to a poor printing resistance.
As for the prolongation of the life of a developer, investigations must be made with consideration being given to the performances of core particles and a toner. Specifically, where a ferrite carrier is used as core particles, the ratio and homogeneity of a composition composed of raw materials of ferrite, such as Fe.sub.2 O.sub.3, NiO, CuO, CoO, MgO, ZnO, MnCO.sub.3, BaCO.sub.3, SrCO.sub.3, Li.sub.2 (CO.sub.3) and CdO, are important, and materials little liable to undergo any chemical change must be selected.
Meanwhile, a toner, which is generally prepared by kneading a thermoplastic resin and a dye or pigment as the main components, pulverizing the kneaded mass, and classifying the resulting powder to provide an optimum particle size distribution, is mixed with a carrier to be ready for use. The characteristics of the resulting developer largely depend on the performance of the resin used. Where use is made of a resin having a low softening point and hence a low molecular weight, the resulting toner is so liable to stick to the carrier and so readily crushable that not only the life of the resulting developer is conspicuously shortened but also the toner fuses and adheres to a heat roller in the case of heat roller fixing to cause hot offsetting to thereby notably deteriorate an image. However, a mere increase in the molecular weight makes a resin tough to lower the fixability of a toner on supports such as paper to deteriorate the overall performance thereof, though it may prolong the life of a developer.
An object of the present invention is to solve the above-mentioned problems of the prior art to thereby provide a developer for electrostatic images which has a sufficient durability, a stable frictional chargeability and a markedly long life.
Another object of the present invention is to provide a developer for electrostatic images which is excellent in flowability and free of the dependence on environment.
Still another object of the present invention is to provide a developer for electrostatic images which is excellent in fixabilty on supports such as paper and resistance to offset onto a roller.
A further object of the present invention is to provide a developer for electrostatic images which enables high-quality image printing and duplication with a good resolution, a good tone reproduction and a low background density.